New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea /sɛˈbɔɪdiːə/, the only extant superfamily in the parvorder Platyrrhini /plætɪˈraɪnaɪ/.[3]
Platyrrhini means broad-nosed, and their noses are flatter than those of other simians, with sideways-facing nostrils. Monkeys in the family Atelidae, such as the spider monkey, are the only primates to have prehensile tails. New World monkeys' closest relatives are the other simians, the Catarrhini ("down-nosed"), comprising Old World monkeys and apes. New World monkeys descend from African simians that colonized South America, a line that split off about 40 million years ago.[4]
About 40 million years ago, the Simiiformes infraorder split into the parvorders Platyrrhini (New World monkeys—in South America) and Catarrhini (apes and Old World monkeys—in Africa).[5] The individuals whose descendants would become Platyrrhini are currently conjectured to have migrated to South America either on a raft of vegetation or via a land bridge. There are two possible rafting routes, either across the Atlantic Ocean from Africa or across the Caribbean from North America; however, there is no fossil record to support the hypothesis of a migration from North America. The land bridge hypothesis relies on the existence of Atlantic Ocean ridges and a fall in the sea level in the Oligocene. This would have produced either a single land bridge or a series of mid-Atlantic islands to act as stepping stones for the migration. The latter is now the favoured land bridge hypothesis.[6]
At the time the New World monkeys split off, the Isthmus of Panama had not yet formed, ocean currents and climate were quite different, and the Atlantic Ocean was less than the present 2,800 km (1,700 mi) width by about a third; possibly 1,000 km (600 mi) less, based on the current estimate of the Atlantic mid-ocean ridge formation processes spreading rate of 25 millimetres per year (1 in/year).[citation needed]
However, the non-platyrrhini Ucayalipithecus of Amazonian Peru who might have rafted across the Atlantic between ~35–32 million years ago, are nested within the Parapithecoidea from the Eocene of Afro-Arabia.[7] Parvimico and perupithecus from Peru appear to be at the base of the Platyrrhini.[8]
The chromosomal content of the ancestor species appears to have been 2n = 54.[9] In extant species, the 2n value varies from 16 in the titi monkey to 62 in the woolly monkey.